The field of the invention relates to a method of preparing catalyst particles and then depositing those particles onto an inert support, and to the products of that method.
Catalysts are used in a variety of industrial chemical processes to speed the approach of a chemical reaction to equilibrium. In the reaction, a true catalyst is not consumed as one of the reactants in the reaction; its role is to facilitate the progress of the reaction. As such, the catalyst is used over and over again in a chemical reaction. In order for the catalyst to be used in a manner where it is available to the reactants in a continuous chemical process, the catalyst is generally supported on an inert substrate that acts primarily as a mechanical support. The reactants in a chemical process are exposed to the catalyst at temperature and pressure conditions that are favorable for the reaction to proceed, and the products are extracted. Often, the reactants flow over a bed of the supported catalyst in a reactor and the products are formed continuously, flowing out of the reactor.
Many of the chemical compounds that are used for catalysts are expensive precious metals. Commonly used industrial catalysts include silver, platinum, palladium, rhodium, cobalt, iron, and nickel and oxides thereof. Other transition metals, transition metal oxides, mixed metal oxides, and lanthanide metals and oxides may also be used. The scale of chemical reactions carried out in modem industry is often large, requiring large quantities of expensive catalytic compounds. Because of the high cost of catalysts, and the general goal of making chemical reactors efficient, it is generally desirable to make catalysts as efficient as possible. For instance, colloidal forms of metals by themselves can act as catalysts, but are of little value in any commercial process since recovery of the catalyst is a problem. Catalysts are therefore usually supported on materials of large surface area and porosity known as catalyst supports. Efficiency of catalytic activity typically requires a large surface area and demands uniformity of the catalyst on the support.
Examples of the industrial use of catalysts include the reaction of hydrogen with sulfur compounds found naturally in crude oil to form hydrogen sulfide. In this case, platinum and cobalt-molybdenum catalysts are used to enhance the rate of reaction between the hydrogen and the sulfur containing compounds found in the oil so that the reaction takes place at an acceptable rate. The hydrogen sulfide is separated from the oil leaving the oil with a reduced sulfur content. The catalyst remains in the reactor so that it can be used over again in the reaction.
In general, the activity of a catalyst depends on the surface area of the catalyst since the reactants must come together in the presence of the catalyst before a reaction can take place. For a given mass of a specific catalyst, the most efficient form of the catalyst is where it has the largest surface area, so its surface is accessible to the reactants. Thus in the preparation of catalysts, it is desirable to deposit the catalyst on a solid support so that it has a maximal surface area.
It is recognized in the prior act that there are two important methods of chemical synthesis of catalysts on a support. The first, termed the precipitation method, involves mixing two or more solutions or suspensions of material resulting in precipitation of the catalyst. Following reaction the resulting material is washed, filtered, dried, and heated. In the second method, known as the impregnation method, the support is contacted with a solution of one or more metallic compounds. The support is then dried and the catalyst is activated chemically or physically. Typically this involves heating in a reductive atmosphere or heating to induce decomposition calcination. As an example, platinum chloride can be adsorbed on dry, particulate aluminum oxide. On heating in a hydrogen atmosphere, the platinum chloride decomposes leaving metallic platinum particles on the aluminum oxide. The platinum on the aluminum oxide support may then be used as a catalyst in a reactor.
In some cases, the activities of a catalyst may be further enhanced by the addition of small amounts of a catalyst promoter. For example, in the production of ethylene oxide, silver loaded on an alumina support is used as the catalyst. Small amounts of rubidium or cesium are also added to promote the reaction.
Such purely chemical methods of preparing catalysts on supports give only indirect or marginal control of the particle size distribution (the mean diameters of the particles and the standard deviation in the diameter) of the catalyst particles on the support. Further problems with purely chemical methods include control of the uniformity of distribution of the catalyst on the support.
In addition to the purely chemical methods noted above, there are also known methods for preparing catalysts with the use of lasers. A problem with some known laser teachings is that they only include a metal catalyst as a target in solution. There is no inert support or any other substrate present for the catalyst to attach to or be deposited on. Other references teach absorption of the laser energy by the substrate and not a metal catalyst.
It is an object of the invention to overcome the foregoing drawbacks and provide a method for modifying the size of catalyst particles on a support and/or controlling the size of catalyst particles deposited on a support through absorption of intense pulsed laser radiation. It is also an object of the invention to provide a means of catalyst synthesis, and depositing catalyst on a support as a film, or as particles, using as starting materials bulk metal, metallic compounds, colloidal particles, previously synthesized catalyst on a support, and/or support materials.
The invention uses a pulsed laser beam that is absorbed by catalyst particles. The effect of absorption of the laser radiation is to convert a portion of the mass of the particles into aqueous ions. The ions recombine on the surface of a substrate (or support) as small particles, a film, or small agglomerations of atoms. The unaffected particles in suspension can also deposit on the substrate. The laser interacts more strongly with larger particles than it does with the smaller ones. This is the case whether the particles are in suspension or on the substrate. Thus, large particles in suspension or on the substrate are converted into ions in solution that can form new, small particles, films, or agglomerations of atoms on the surface of the substrate. The ejection of ions from the irradiated particles also leads to a size reduction of the larger particles. The process of conversion to ions, chemical reduction, and deposition on the surface can take place over and over again. The magnitude of the interaction depends not only on the specific particle being irradiated and on the particle diameter, but also on the laser intensity. Thus a method is provided for depositing catalyst particles on a support, controlling the size of catalyst particles, and controlling the distribution of particle sizes on a support.
The deposition of particles on a support is accomplished by irradiating a catalyst suspension, a precursor catalyst compound, or a bulk catalyst. The invention uses a laser to produce a catalyst on a support. The invention provides a means for making smaller and more uniform particles than is commonly produced by chemical methods thereby making the catalyst more efficient. After the irradiation process is complete, the catalyst may be dried and it may be activated by reduction or oxidation methods to obtain the desired active state of the catalyst particles.